Chapter 9 - Perceiving Colour Flashcards
1- Colour blindness and
colour deficiency
Cerebral achromatopsia:
* A type of color-blindness caused by damage to the cerebral cortex of the
brain, rather than abnormalities in the cells of the eye’s retina.
* Damage to the ventro-medial occipital and temporal lobes.
Colour deficiency or Congenital achromatopsia:
* Occurs at birth because of the genetic absence of one or more types of cone receptors.
-Most people who are born partially color blind are not disturbed by their decreased color perception compared to “normal,”
-They have never experienced color as a person with normal color
vision does.
-People can perceive color but have difficulty distinguishing between
certain colors, such as red and green.
2- Functions of Color Perception
Signaling functions
- Help us identify and
classify things.
Ex: banana ripe when yellow, stop when red traffic light
- Perceptual organization.
similar elements grouped together and segregated from their background - The ability to detect colored
food has led to the proposal
that monkey and human
color vision may have evolved for the express purpose of detecting fruit
Ex: image of fruit on plants in colour vs. in black and white (very hard to detect the fruit)
Knowing the colors of familiar objects helps us to recognize these objects.
Ex: banana is yellow, cabbage is green, carrot is orange…
When not in those colours, harder to recognize.
- Color can be a cue to
emotions signaled by facial
expressions. - Participants were more likely to rate the face as expressing disgust when colored green and as expressing anger
when red.
3- Colour and Light
Newton’s experiment (like Pink Floyd album cover)
1. Newton made a hole in a window shade, which let a beam of sunlight enter the room. When he placed Prism 1 in its path, the beam of white-appearing light was split into the components of the visual spectrum.
2. He thought that white light was a mixture of differently colored lights and that the prism separated the white light into its individual
components.
3. Newton next placed a board in the path of the
differently colored beams. Holes in the board
allowed only particular beams to pass through while
the rest were blocked. Each beam that passed through the board then went through a second prism, shown as Prisms 2, 3, and 4, for the red, yellow, and blue rays of light.
(SEE IMAGE, IT REALLY HELPS)
* Newton noticed two important things about the light that passed through the second prism.
- The second prism did not change the color appearance of any light that passed through it. For example, a red beam continued to look red after it passed through the second prism.
- Second, the degree to which beams from each part of the spectrum were “bent” by the second prism was
different.
* Newton concluded that light in each part of the spectrum is defined by different physical properties and that these physical differences give rise to our perception of different colors.
* Wavelengths from about 400 to 450 nm appear violet; 450 to 490 nm, blue; 500 to 575 nm, green; 575 to 590 nm, yellow; 590 to 620 nm, orange; and 620 to 700 nm, red.
Spectral power distribution:
Spectrum of electromagnetic radiation:
(from smallest wavelenght to biggest)
Gamma rays
X-rays
UV rays
Visible spectrum of light (colours=high intensity compared to others)… 400nm to 700nm
Infrared radiation
Micro-waves
Radio waves
(SEE IMAGE)
Reflectance and Transmission
* Selective REFLECTION
- Chromatic colors, such as blue, green, and red, occur when some wave-lengths are reflected more than others.
* Equal reflection of wavelengths:
-Achromatic colors, such as white, gray, and black, occur when light is reflected equally across the spectrum.
* Selective TRANSMISSION
- Selective transmission means that only some wavelengths pass through the object or substance
(SEE IMAGE)
Predominant wavelengths reflected or transmitted and perceived colour:
-short = blue
-medium = green
-long and medium = yellow
-long = red
-long, medium and short = white
Ex: lettuce= higher percentage (50%) of reflectance in medium wave lengths (green)
tomato= higher percentage (60%) of reflectance in long wavelengths (red)
white paper = VERY HIGH (80%) percentage of reflectance in short, medium and long wavelengths. Doesn’t absorb much, reflects!
grey paper = lower percentage (20%) reflectance for short, medium and long wavelengths
black paper = very low (5%) reflectance for short, medium and long wavelengths. Doesn’t reflect much, absorbs!
(SEE IMAGE)
4- Colour and Light : Mixing
Color Mixing
* Mixing Paints
- Subtractive color mixture.
- Mixing yellow and blue paints results in green.
Why is this so?
Blue paint: reflects all SHORT WL, reflects some MEDIUM WL, absorbs all LONG WL
Yellow paint: absorbs all SHORT WL, reflects some MEDIUM WL, reflects some LONG WL
Mixing blue and yellow paint: absorbs all SHORT WL, reflects some MEDIUM WL, absorbs all LONG WL
Medium wavelength = green!!
The key to understanding what happens when colored paints are mixed together is that when mixed, both paints still
absorb the same wavelengths they absorbed when alone, so the only
wavelengths reflected are those that are reflected by both paints in common.
(SEE IMAGES)
Mixing Lights (not the same as paint!)
* Additive colour mixing
- What would happen if we mix together blue and yellow lights?
- If a light that appears blue is projected onto a white surface and a light that
appears yellow is projected on top of the light that appears blue, the area where
the lights are superimposed is perceived as WHITE.
- Added together light therefore contains
short, medium, and long wavelengths.
Summary:
Blue (short wl) + yellow (medium and long wl) = white (short, medium and long)
The key to understanding what
happens when colored lights are
superimposed is that all of the light that
is reflected from the surface by each light
when alone is also reflected when the lights
are superimposed.
(SEE IMAGE)
Summary:
■■ Colors of light are associated with wavelengths in the
visible spectrum.
■■ The colors of objects are associated with which wavelengths
are reflected (for opaque objects) or transmitted
(for transparent objects).
■■ The colors that occur when we mix colors are also associated
with which wavelengths are reflected into the eye.
Mixing paints causes fewer wavelengths to be reflected
(each paint subtracts wavelengths from the mixture); mixing
lights causes more wavelengths to be reflected (each
light adds wavelengths to the mixture).
5- Perceptual Dimensions
of Colors
- Spectral colors
- Colors evoked by monochromatic light (violet, (sometimes indigo), blue, green, yellow, orange, red)
(i.e., a pure wavelength of light). - Non-spectral colors
- Colors that do not appear in the spectrum because they are mixtures of other colors, such as magenta (a mixture of blue and red).
- How many colours?
- A conservative estimate is that humans can tell the difference between about
2.3 million different colors
3 perceptual dimensions
1. Hues/chromatic colours
- red, orange, yellow, green, blue, violet
2. Saturation
- Refers to the intensity of color.
-when more white added to color, saturation decreases: when desaturated, take on faded or washed-out appearance
3. Value/lightness
- The light-to-dark dimension of color.
* Colour solid
-3d color space
- Illustrates the relationship among hue, saturation, and value
Munsell colour system
6- The Munsell colour
system
Saturation, value and hue
(SEE IMAGE)
Different hues are arranged around the circumference of
the cylinder with perceptually similar hues placed next to each
other. Notice that the order of the hues around the cylinder
matches the order of the colors in the visible spectrum shown
in Figure 9.4b. Saturation is depicted by placing more saturated
colors toward the outer edge of the cylinder and more
desaturated colors toward the center. Value is represented
by the cylinder’s height, with lighter colors at the top and
darker colors at the bottom. The color solid therefore creates a
coordinate system in which our perception of any color can be
defined by hue, saturation, and value.
Draw image, it will help
The Munsell color space. Hue is arranged in a circle around the vertical, which represents value. Saturation increases
with distance away from the vertical.
7- The Trichromatic Theory of Color Vision
Newton’s proposal
- Each component of the
spectrum must stimulate the
retina differently in order for
us to perceive color.
Young-Helmholtz theory
* Color vision based on three
principal colors
* Three different receptor
mechanisms
*Marks the birth of what is today
called the trichromacy of color vision, which in modern
terminology states that color vision depends on the activity
of three different receptor mechanisms
Behavioral evidence:
* Color-matching experiments (Maxwell)
- Participants with normal color vision
- Adjusted amounts of different wavelengths in a comparison field to match a test field of one wavelength.
* The experiment
- 1) The experimenter presents a reference color that is created by shining a single wavelength of light on a “reference field”.
- 2)The observer then matches the reference color by mixing different wavelengths of light in a “comparison field”.
* In this example, the observer is shown a 500-nm light in the reference
field on the left and then asked to adjust the amounts of 420-nm, 560-nm, and 640-nm lights in the comparison field on the right, until the
perceived color of the comparison field matches the reference field.
* Results
- Any reference color could be matched provided that observers were able to adjust the proportions of three wave-lengths in the
comparison field.
- Two wavelengths allowed participants to match some, but not all, reference colors.
* They never needed four wavelengths to match any reference color.
* Conclusion
- Colour vision depends on three receptor mechanisms, each with different spectral sensitivities
(SEE IMAGE)
Researchers measured absorption spectra of
visual pigments in receptors (1960s).
* They found pigments that responded (absorbed) maximally to:
- Short wavelengths (419nm)
- Medium wavelengths (531nm)
- Long wavelengths (558nm)
Later researchers found genetic differences for coding proteins for the three pigments (1980s).
By presenting light at wavelengths across the spectrum, it was determined that there were three types of cones, with the absorption spectra shown in Figure 9.13.
8- Cone Responding
and Color Perception
Color perception is based on the response of the three different types of cones.
* Microspectrophotometry
- A technique used to measure the absorption or
transmission spectrum of a solid or liquid material in
either transmitted or reflected light.
- Responses vary depending on the wavelengths
available.
- Combinations of the responses across all three cone types lead to perception of all colors.
- Color matching experiments show that colors that are
perceptually similar, (metamers) can be caused by
different physical wavelengths.
Measuring the Characteristics of the Cone
Receptors
* Adaptive optical imaging
- Taking pictures that show how the cones are arranged
on the surface of the retina.
* Cone mosaic
Ex:
Blue: bigger short wl cones
Green: bigger medium wl cones
Yellow: bigger medium and long wl cones
Red: bigger long wl cones
white: short, medium and long wl cones are big
(see image)
I think that’s what it means???? (READ AND VIDEO)
Patterns of firing of the three types of cones to
wavelengths associated with different colors. The size of the cones
symbolizes the amount of activity in the short-, medium-, and longwavelength
cones.
- Metamerism:
This situation, in
which two physically different stimuli are perceptually identical,
is called metamerism, and the two identical fields in a
color-matching experiment are called metamers. - Left is a metamere (see image)
Figure 9.16 Principle behind metamerism. The proportions of
530-nm and 620-nm lights in the field on the left have been adjusted
so that the mixture appears identical to the 580-nm light in the field
on the right. The numbers indicate the responses of the short-,
medium-, and long-wavelength receptors. There is no difference in
the responses of the two sets of receptors, so the two fields are
perceptually indistinguishable.
Other way of saying it:
The reason metamers look alike is that they both result
in the same pattern of response in the three cone receptors.
For example, when the proportions of a 620-nm red light that
looks red and a 530-nm green light that looks green are adjusted
so the mixture matches the color of a 580-nm light,
which looks yellow, the two mixed wavelengths create the
same pattern of activity in the cone receptors as the single 580-
nm light (Figure 9.16). The 530-nm green light causes a large
response in the M receptor, and the 620-nm red light causes
a large response in the L receptor. Together, they result in a
large response in the M and L receptors and a much smaller
response in the S receptor. This is the pattern for yellow and is
the same as the pattern generated by the 580-nm light.
even though the lights in these two fields are physically different,
the two lights result in identical physiological responses
so they are identical as far as the brain is concerned and they
are therefore perceived as being the same.
9- Are Three Receptor
Mechanisms Necessary for
Color Vision?
One receptor type cannot lead to color vision
* Monochromatism
- Rare form of color blindness that is usually
hereditary and occurs in only about 10 people out of 1 million.
- Monochromats:
- No functioning cones, so their vision is created only by the rods.
Their vision, therefore, has the characteristics of rod vision in
both dim and bright lights so they see only in shades of lightness
(white, gray, and black) and can therefore be called color
blind.
* Principle of univariance (for monochromacy)
- Absorption of a photon causes the same effect, no matter what the wavelength is.
-states that once
a photon of light is absorbed by a visual pigment molecule,
the identity of the light’s wavelength is lost.
Ex: wl of 480nm on 1,000 photon intensity vs. wl of 600nm on 1,000 photon intensity. Which appears brighter? Both appear identical.
- Any two wavelengths can cause the same response by changing the intensity.
Ex: wl of 480nm on 100 photon intensity vs. wl of 600nm on 50 photon intensity. Which appears brighter? 480
(SEE IMAGE)
Two receptor types (dichromats) solve this problem, but three
types (trichromats) allow for the perception of more colors.
Principle of Univariance
(see image and vid)
Figure 9.17 (a) Absorption spectrum of a visual pigment that
absorbs 10 percent of 480-nm light and 5 percent of 600-nm light.
(b) Visual pigment molecules isomerized when the intensity of both
480-nm and 600-nm lights is 1,000, determined by multiplying
intensity times the percent of light absorbed. Because more visual
pigments are isomerized by the 480-nm light, it will appear brighter.
(c) Molecules isomerized when the intensity of the 480-nm light is
1,000 and the intensity of the 600-nm light is 2,000. In this case,
both lights will look identical.
What this means is that a person with only one visual pigment
can match any wavelength in the spectrum by adjusting
the intensity of any other wavelength and sees all of the wavelengths
as shades of gray. Thus, adjusting the intensity appropriately
can make the 480-nm and 600-nm lights (or any other
wavelengths) look identical.
Vision With Two Receptor Types
People may have deficiencies in color perception.
* Monochromat: just one type of pigment
- see shades of grey
* Dichromat: two types of pigment
- see chromatic colors, but cannot distinguish among all colors
* Trichromat: three types of pigment
- can discriminate among more wavelengths across the spectrum
Three major forms of
dichromatism
* There are three major forms of dichromatism
(see image comparing all 3)
- Protanopia
* Affects 1 percent of males and 0.02 percent of
females
* A protanope is missing the long-wavelength pigment.
* Protanope perceives short-wavelength light as blue, and as the wavelength is increased, the blue becomes less and less saturated.
* At 492 nm the protanope perceives gray
* The wavelength at which the protanope perceives gray is called the neutral point.
* At wavelengths above the neutral point, the protanope perceives yellow, which becomes less intense at the long wavelength end of the
spectrum. - Deuteranopia
* Affects about 1 percent of males and 0.01 percent of females
* A deuteranope is missing the mediumwavelength pigment.
* A turquoise perceives blue at short wavelengths, sees yellow at long wavelengths, and has a neutral point at
about 498 nm. - Tritanopia
* Very rare, affecting only about 0.002 percent of males and 0.001 percent of
females.
* A tritanope is missing the shortwavelength pigment.
* A tritanope sees colors as in Figure 9.20d and sees the spectrum as in Figure 9.21d
—blue at short wavelengths, red at long wavelengths, and a neutral point at 570 nm.
Anomalous trichromatism
* An anomalous trichromat needs three wavelengths to match any wavelength, just as a normal trichromat does.
* However, the anomalous trichromat mixes these wavelengths in different
proportions from a trichromat, and an
anomalous trichromat is not as good as a trichromat at discriminating between
wavelengths that are close together.
10- The Opponent-Process Theory of Color Vision
Proposed by Hering (1800s)
* Color vision is caused by opposing responses
generated by blue and yellow, and by green
and red.
*Behavioral evidence:
- Color afterimages and simultaneous color
contrast show the opposing pairings.
- Types of color blindness are red/green and blue/yellow.
Ex: american flag negative afterimage
Opponent-process mechanism proposed by Hering
* Three mechanisms: red/green, blue/yellow, and
white/black
* The pairs respond in an opposing fashion, such as
positively to red and negatively to green.
* These responses were believed to be the result of
chemical reactions in the retina.
* Primary colors
- Red, yellow, green, and blue—and proposed that
each of the other colors are made up of combinations of these primary colors.
* Hue scaling
- participants were given colors from around the
hue circle and told to indicate the proportions of
red, yellow, blue, and green that they perceived in
each color. One result was that each of the primaries was “pure.”
Complementary afterimages
- Red and green switch places and blue and yellow
switch places.
- Hering’s opponent-mechanism proposal wasn’t
widely accepted - (1) its main competition, trichromatic theory, was
championed by Helmholtz, who had great prestige
in the scientific community. - (2) Hering’s phenomenological evidence, which was based on describing the appearance of colors, could not compete with Maxwell’s quantitative color mixing data.
- (3) there was no neural mechanism known at that
time that could respond in opposite ways.
Evidence for the Opponent-Process Theory
Psychophysical Evidence:
* Hue cancellation experiments
- Leo Hurvich and Dorthea
Jameson’s (1957)
- Provided quantitative
measurements of the
strengths of the B–Y and
R–G components of the
opponent mechanisms.
* 1) We begin with a 430-nm light, which appears blue.
* 2) Since yellow is the opposite of blue and therefore cancels it, they could determine the amount of blueness in a 430-nm light by determining how much
yellow needs to be added to cancel all perception of
“blueness.”
* 3) The blue dot in Figure 9.23 indicates the amount of
yellow that was added to 430-nm light to cancel all
“blueness.”
* 4) Once this is determined for the 430-nm light, the
measurement is repeated for 440 nm and so on, across the spectrum, until reaching the wavelength where there is no blueness, indicated by the circle.
Evidence for the Opponent-Process Theory
Researchers performing single-cell recordings found
opponent neurons (1950s)
* Opponent neurons:
- Are located in the retina and LGN.
- Respond in an excitatory manner to one end of the
spectrum, and an inhibitory manner to the other.
11- How Opponent Responding Can Be
Created by Three Types of Receptors
Each theory describes the physiological mechanisms in the visual system.
* Trichromatic theory explains the responses of the cones in the retina.
* Opponent-process theory explains neural response for cells connected to the cones
further in the brain.
